1. Field of the Invention
Our present invention relates to optical filter coatings and to processes for making such coatings; to red, orange and yellow dichroic filters; to processes for coating such filters on flexible substrates and on lamp envelopes; and to red, orange and yellow bulbs used, for example, in emergency lights, and in automobile tail lights, turn signal lights and side marker lights.
2. Description of the Related Art
It is a conventional practice in the optical coating industry to form red, orange or yellow dichroic filters of a periodic stack of alternating low index (L) and high index (H) layers of transparent dielectric materials having a relatively low index ratio of about 2.2/1.46. Using the dielectric materials SiO.sub.2 or MgF.sub.2 (L) and TiO.sub.2, Ta.sub.2.sup.0 5, ZnS, ZrO.sub.2, HFO.sub.2, etc. (H), the two stack construction and (H/2 L H/2).sup.n at .lambda..sub.2, with n =6, 7 or 8 and positioning .lambda..sub.1 and .lambda..sub.2 appropriately in the visible spectrum, one can position the cut-on wavelength (the wavelength between regions of high reflectance and high transmittance) to yield an appropriate transmitted color, i.e., to reflect unwanted wavelengths and transmit the wavelength of the desired color. For yellow filters, typically only one stack of about 16 layers is required. However, the necessary layer count is about 25 for orange filters and about 30-40 for red filters. Because of this large number of layers, these filters are time consuming and expensive to build.
In addition, these filters typically have high intrinsic stress due to the high layer count. The high stress levels make these types of filters unsuitable to coating on such substrates as flexible plastic films. These filters also are very sensitive to incidence angle because of the property of all dielectric materials to shift to shorter wavelengths with increased angles. The sensitivity to high incidence angles occurs for example in lamp envelopes having a red or orange dielectric dichroic filter coating. Characteristically, blue-green light is reflected internally until the light strikes the envelope and filter at a high angle, at which point the light is transmitted through the filter and degrades/desaturates the orange or red color. Because of the stress inherent in coatings containing as many as 30 layers (red filters), and as many as 40-45 layers (blue-corrected red filters), and because of the sensitivity to high incidence angles, all-dielectric filters are difficult or impossible to build on substrates such as lamp envelopes or flexible substrates, and provide unacceptable performance and cost.
N.V. Philips German Patent Document 8600642 (Rollennummer) discloses a blue energy. absorbing yellow lamp filter design which incorporates a single very thin blue-absorbing layer on the lamp envelope and many all dielectric periods on top to reject light. The all dielectric filter coating comprises the conventional long wave stack (H/2 L H/2).sup.n, where n = 5 or 6. and, where L is SiO.sub.2, and H is a material such as TiO.sub.2 or Ta.sub.2 O.sub.5. The absorbing material is very thin, 5-6 nm, to prevent loss of visible light output while attenuating only the blue region of the visible spectrum.
However, any attempt to achieve the automobile industry standards for color purity for orange or red using the 8600642 design approach would still require up to 30 layers on top of the absorbing material. It is submitted that even such a many-layered coating could not achieve the purity required to meet the industry standard for these colors because of light leakage in the green region of the spectrum. Regardless, even if such purity were attainable, the fabrication process would be lengthy and not cost effective and the resulting coating would be extremely angle sensitive. In short, it is believed the filter would not be suitable for lamp applications.